A simple method has been developed to measure the extent of toxicity by measuring the leakage of the label into the medium from myocytes whose cell ATP pool has been prelabeled by incubating them with C14-adenine. By this method calcium ionophore A23187 was shown to induce a dose dependent toxicity in myocytes in culture. An uncoupler of oxidative phosphorylation such as 2,4-dinitrophenol which did not produce any toxicity in these cells potentiated the toxicity induced by ionophore A23187 indicating that uncoupling of mitochondrial oxidation phosphorylation is not alone sufficient to lead to cardiotoxicity.